Higher excited states of benzene: Symmetry assignments of six gerade Rydberg series by four-photon absorption spectroscopy

We report the observation of six gerade Rydberg series in benzene converging to the first ionization potential, seen in the region from 14 500-18 500 cm^-1 as four-photon resonances in the five-photon ionization spectrum of the jet-cooled molecule. Polarization ratios measured for each transition in two- as well as four-photon resonance aid in determining excited state symmetries. We assign a Rydberg series with a quantum defect of 0.76 to higher members of the 3s Rydberg state first seen by Johnson in the near UV two-photon resonant three-photon ionization spectrum of benzene. The observed quantum defect, polarization ratios, and correlation with synchrotron absorption data all strongly support this assignment. Another series, with a quantum defect of 0.24, exhibits a polarization ratio of 4.5±0.5 signifying pure rank four activity. This is the first report of a purely rank four transition in a molecule. We assign this as an excitation to a d₂ orbital which when converging to the first ionization potential (the ²E _1g cation) gives rise to B_1g and B_2g Rydberg transitions. Within the D_6h point group, these symmetries are allowed only in a four-photon transition. The number of gerade Rydberg series observed as four-photon resonances exceeds those available from the s-(1) and d-(3) bound members of a hydrogenic basis set. The participation of g(l=4) orbitale is indicated as the lowest members of two of the Rydberg series are at the n=5 level.